There exists a need for a high tensile strength steel which when subjected to a high neutron flux over an extended period of time will not swell significantly and which will not become so highly radioactive that it cannot be disposed in surface sites. A particular application for such a steel would be a first-wall or a blanket in a fusion reactor.
Steels currently available which meet the strength and swelling requirements are unsuitable for surface disposal because they have unacceptable levels of elements that transmutate upon neutron bombardment to particularly problematic nuclides, such as those having very long half-lives.
Activated materials require removal to and storage at a remote location whereat the released radiation will have a negligible effect on people or the environment. Currently, the lightwater reactor (LWR) waste guideline, 10 C.F.R. 61, has three classes of waste; classes A, B and C, which are all surface disposal. All others not meeting this are only considered for geologic disposal. All conventional steel alloys will activate such that they would not qualify for surface waste disposal after long-term and/or high level exposure to neutron flux, and would require deep, geologic disposal for many generations because of major constituent elements or levels of impurities too high for surface disposal. All elements will activate upon neutron bombardment, and only a relatively few elements have daughter radionuclides that decay quickly enough or are weak enough such that they could be disposed of as surface waste. The relative costs of disposal methods cannot be estimated easily, but for packaging and burying without monitoring, costs have been recently estimated at $200-600 per cubic foot for any of the three classes of surface waste, and at about $200,000 per cubic foot for the only other alternative, geological disposal.
Wastes represent a potential safety risk in that activated species could be released to the surrounding environment during a reactor accident or, after disposal, by natural deterioration processes. Currently, the most important mechanisms for release of activated nuclides are through lithium or other breeder material fires during normal operating service and by loss of coolant to highly activated material which has its own heating due to radioactive decay, Holdren, J., Science 200:168 (1978). Both mechanisms raise the temperature of the material, potentially resulting in vaporization of activated nuclides. Although high melting point elements have relatively little tendency to volatize, they may form surface oxides which volatize at temperatures well below the melting point of the unoxidized material. From the standpoint of volatization, manganese is particularly undesirable as its major daughter nuclide is another isotope of manganese, .sup.54 Mn, and manganese itself has a relatively high vapor pressure and also forms a volatile oxide.
One solution that has been proposed for eventual disposal of materials subjected to neutron bombardment is "isotopic tailoring" which is the removal of certain naturally occurring isotopes from alloying elements. For example, molybdenum has nine stable occurring isotopes, two of which .sup.94 Mo and .sup.95 Mo capture neutrons, activating to unacceptable radionuclides .sup.93m Nb and .sup.93 Mo, respectively. Isotopic tailoring would cause .sup.94 Mo and .sup.95 Mo to be removed during isotopic processing such that the dominant radionuclides .sup.93m Nb and .sup.93 Mo could not be produced. The disadvantages of isotopic tailoring is that an entire industry would have to be created to separate the offending isotopes in every alloying element addition, and this would be an enormous task. Furthermore, residual impurity levels must be very small, and it is not clear that isotopic tailoring on a large scale could produce elements with the required controlled levels of offending isotopes.
Another proposed solution is to use conventional materials, such as AISI 316 austenitic stainless steel, without offending elements, but this also has two disadvantages. The problem of impurity control is also a factor here, and it would be difficult to overcome, although with judicious selection of the alloy system, certain impurities might be minimized. However, this class of steels uses large additions of alloying elements to achieve their "austenitic" characteristics, and because certain alloying element additions may introduce too high levels of impurities, the austenitic class may never achieve the low residual impurity requirement. Furthermore, it is established that austenitic steels increase in volume during exposure to a neutron flux, and swelling of austenitic steels is presently considered to be outside the acceptable design limits for dimension changes in nuclear reactors.
Certain martensitic steels are known which satisfy the strength requirements for use as a first-wall or blanket material for a fusion reactor, and because of its body-centered cubic microstructure, the martensitic form of steel does not tend to swell beyond acceptable limits. Unfortunately, the two common elements used in martensitic steel, molybdenum and nickel, transmutate into daughter nuclides which in the quantities almost certain to be generated, would be highly unacceptable for surface disposal. Molybdenum is used for strength and stability of the microstructure while nickel is used for increased toughness and hardenability (i.e. creating the "martensitic" structure).
Although martensitic steels are known which use substitutes for molybdenum and nickel, simple exclusion of problematic transmutagenic elements from the composition formula is insufficient to render a steel alloy surface-disposable. Impurities in all steels manufactured by known techniques inherently incorporate impurities at levels which would make these materials unsuitable for surface disposal after long-term exposure to neutron flux.
It would be desirable to have a martensitic steel having requisite strength for nuclear reactor use and have sufficiently low concentrations of elements which transmutate into those daughter nuclides for which very low concentrations are permissible for surface-disposal.